It has been proposed heretofore to provide an apparatus for contacting material to be deep frozen with cryogenic coolant, generally a liquefied gas such as liquid nitrogen, to effect rapid bulk reduction in temperature as part of a preserving, embrittling or like process.
These techniques have been used, for example, for embrittling materials such as elastomers and synthetic resins in conjunction with a comminuting step enabling the materials to be reused, for the deep freezing of comestibles for preserving them directly or in conjunction with freeze drying, and for the preserving of biological specimens with a minimum of cell denigration.
A spraying system which utilizes an insulated freezing chamber with a transport means for the material to be deep frozen is described, for example, in U.S. Pat. No. 4,103,507 issued Aug. 1, 1978.
This system provides an insulated freezing chamber through which the product to be contacted with the sprayed cryogen is introduced through an inlet opening and is transported to an outlet opening. Conveying means is provided to effect the displacement of the material through the chamber while above the transport path a distribution duct or pipe is provided with a multiplicity of spray nozzles.
Liquefied nitrogen is sprayed upon the product. The distribution duct or pipe forms, for example, a double loop which spans a relatively long portion of the transport path.
An important aspect of this system is that the spray zones of the nozzles overlap so that the total area occupied by the material is completely covered by the spray.
This spray system has, however, various disadvantages. For example, the long length of the distribution or manifold pipe permits a relatively large heat transfer from the freezing chamber to the distribution pipe and hence to the liquid nitrogen therein. As a consequence, the liquid nitrogen is converted from a single liquid phase to a two-phase mixture of gas and liquid.
The cooling capacity of a gas phase is significantly less than that of a liquid phase so that the discharge of the gaseous coolant into the chamber or against the material to be deep frozen can give rise to a slower cooling period operation.
In most of the cases described above, it is important to obtain a maximum rate of temperature reduction as it is to achieve the final reduced temperature and hence this earlier system is not fully satisfactory.
Efforts to overcome this disadvantage have been made and have involved, for example, efforts to reduce the quantity of gas which will be entrained with or will be present in the liquid phase. These systems effectively tend to limit the amount of a gas phase present in the liquid phase but have not been able to exclude the gas phase.
As a result of the substantially greater volume of the gas phase by comparison to the liquid phase, earlier systems have the additional disadvantage that the discharge through the nozzle of the liquid phase is disturbed; the throughput is disturbed especially where the nozzle throughput is low and the spray area of nozzle is restricted. In fact, the presence of a large-volume gas phase can also reduce the throughput of the liquid phase.
Another problem with earlier systems is that it is difficult to maintain long manifold or distribution pipes in precisely horizontal orientations. The precise horizontal lay of such pipes is especially important for the discharge of a liquid coolant containing a significant quantity of a gaseous phase. If a pipe of this type is not maintained precisely horizontal, the gas phase tends to accumulate at the higher locations of the distribution pipe and to interfere with a uniform discharge of the liquid phase. The cooling capacity of this system is thereby reduced. In extreme cases, only the gas phase may be discharged through the nozzles at the higher side of the distribution pipe assembly while only the liquid phase is sprayed from the remaining nozzles and uniformity of cooling is impossible to achieve. Frequently these earlier systems, in spite of the relatively high cost, large number of nozzles, and considerable spray area, cannot achieve a reproducible, satisfactory or complete deep freezing of the product.